By Susan Milius
Rescue squad
Living among coral branches, the broad-barred goby (Gobiodon histrio) will react to a chemical cue from its sheltering coral releases if touched by a brilliant green, toxic seaweed (right) and nip back the encroaching foliage.
Danielle Dixson

When a killer seaweed touches a kind of spiky coral, the coral pushes a chemical panic button that brings small resident fish to the rescue.

Unchecked, seaweed algae can overrun a coral reef, as the community dwindles in “a descent into slime,” says marine ecologist Mark Hay of the Georgia Institute of Technology in Atlanta. But within 15 minutes of contact with a toxin-making seaweed, an Acropora nasuta coral releases compounds that prompt goby fish to seek out and trim back the seaweed, Hay and colleague Danielle Dixson report in the Nov. 9 Science.

“We’ve lost about 80 percent of the living coral in the Caribbean and 50 percent in the western Pacific,” says coral biologist Nancy Knowlton of the Smithsonian Institution in Washington, D.C. “So a better understanding of what keeps corals healthy is essential.”

In reefs, corals and the seaweed algae that form lawns or shrubby thickets compete for light and space. As coral reefs decline from pollution, overfishing, climate change and other insults, biologists have seen swaths of seaweed take over. Lush seaweed intrusions repel or smother larval corals, accelerating what Hay calls the reef “death spiral.”

In this scenario, corals dwindle, and fish and other reef creatures find fewer safe nooks and crannies to live in. The reef then has fewer algae-eating fish, which means even less protection for the habitat-maker corals that maintain diverse life. “Without these,” Hay says, “you have these algae-covered parking lots.”

The discovery of A. nasuta’s on-demand guards came from thinking small. Hay and Dixson partially caged individual A. nasuta, which branch “like a tightly compacted deer’s antlers,” he says. For each coral, researchers either removed, or left alone, some of the creatures that lurk in the coral’s crannies. Then the researchers tested reactions to the emerald-green seaweed Chlorodesmis fastigiata, one of the reef invaders most toxic to corals.

“It’s a really pretty and really nasty little plant,” Hay says. If left unchecked, fronds that grow against a coral secrete chemical weapons that start killing coral tissue two days after contact.

When researchers fastened toxic seaweed strands to dangle against the corals, two kinds of damselfishes were no help at all, abandoning the troubled sites within 48 hours. Two species of the small colorful gobies that settle into corals, however, “were like little hedge trimmers,” Hay says.

Both the broad-barred goby (Gobiodon histrio) and the redhead goby (Paragobiodon echinocephalus)chewed back seaweed until it no longer brushed against their home coral. The trigger was a substance produced by the coral itself, the researchers found. Gobies ignored seaweedlike tufts of string brushing their corals, unless the string had been treated with water collected beside recently attacked coral. Overall, corals caged with their vigilant gobies suffered only about a quarter of the damage inflicted by seaweed on corals without a goby squad.